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Biomedical subjects

H Ripps

Publications and source records attributed to H Ripps.

108 records · Page 6Linked to original sources
PubMed1970

Visual adaptation in the retina of the skate.

The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10-15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70-90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20-25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.

Adaptation, Ocular↗
PubMed1969

Flash bleaching of rhodopsin in the human retina.

1. Measurements were made of the photo-sensitivity of rhodopsin in the living human retina following bleaching with (a) a flash less than 1 msec in duration, (b) a continuous exposure lasting 30 sec.2. The apparatus consisted of a fundus reflectometer coupled to a computer for on-line processing. Three subjects were studied.3. The relations between the quantity of pigment bleached and the incident energy are so similar for the flash and continuous exposure tests that the occurrence of photo-reversal (such as has been reported in similar circumstances for visual pigment in solution) seems to be ruled out.4. Nonetheless, the two experimental conditions lead qualitatively to two different photo-chemical states in as much as the density difference spectrum for the flash exposure is displaced toward longer wave-lengths in comparison with the data obtained with the continuous exposure.5. The results are discussed with reference to the sequence of events following the bleaching of visual pigments solutions as reported by other workers.

Fundus Oculi↗
PubMed1969

Color vision.

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Animals↗
PubMedDate not supplied

Gap-junctional properties of electrically coupled skate horizontal cells in culture.

Whole-cell voltage-clamp recordings were used to examine the unusual pharmacological properties of the electrical coupling between rod-driven horizontal cells in skate retina as revealed previously by receptive-field measurements (Qian & Ripps, 1992). The junctional resistance was measured in electrically coupled cell pairs that had been enzymatically isolated and maintained in culture; the typical value was about 19.92 M omega (n = 45), more than an order of magnitude lower than the nonjunctional membrane resistance. These data and the intercellular spread of the fluorescent dye Lucifer Yellow provide a good indication that skate horizontal cells are well coupled. The junctional conductance between cells was not modulated by the neurotransmitters dopamine (200 microM) or GABA (1 mM), nor was it affected by the membrane-permeable analogues of cAMP or cGMP, or the adenylate cyclase activator, forskolin. Although resistant to agents that have been reported to alter horizontal-cell coupling in cone-driven horizontal cells, the junctional conductance between paired horizontal cells of skate was greatly reduced by the application of 20 mM acetate, which is known to effectively reduce intracellular pH. Together with the results obtained in situ on the receptive-field properties of skate horizontal cells, these findings indicate that the gap-junctional properties of rod-driven horizontal cells of the skate are fundamentally different from those of cone-driven horizontal cells in other species. This raises the possibility that there is more than one class of electrical synapse on vertebrate horizontal cells.

Animals↗
PubMedDate not supplied

The rhodopsin cycle is preserved in IRBP "knockout" mice despite abnormalities in retinal structure and function.

In the vertebrate retina, vision is initiated and maintained by the photolysis and regeneration, respectively, of light-sensitive pigments in the disk membranes of the photoreceptor outer segments. This cyclical process depends on an exchange of retinoids between the photoreceptors and the retinal pigment epithelium (RPE). There is a great deal of indirect evidence that the transport of retinoids between these cellular compartments is mediated by the interphotoreceptor retinoid-binding protein (IRBP), a large glycoprotein synthesized in the photoreceptors and extruded into the interphotoreceptor matrix (IPM) that fills the subretinal space. Nevertheless, a number of in vitro experiments have demonstrated that an intermembranous transfer of retinoids can occur through an aqueous medium independent of any retinoid-binding protein. This led to the suggestion that IRBP may play the more passive role of an extracellular buffer, serving to prevent the degradation and potentially cytotoxic effects of free retinoids when large amounts are released into the IPM. We have studied the structural and functional properties of transgenic mice in which homologous recombination was used to delete the IRBP gene. Light- and electron-microscopic examination of the retinas of "knockout" (IRBP-/-) mice revealed a significant loss of photoreceptor nuclei, and profound changes in the structure and organization of the receptor outer segments. Consistent with these observations, electroretinographic recordings showed a marked reduction in response amplitude for both rod- and cone-mediated potentials. However, despite the histological and electrophysiological changes, there was no evidence of gross abnormalities in the visual cycle. After bleaching a significant fraction of the available rhodopsin, electroretinogram amplitude and rhodopsin density gradually increased toward their pre-bleach levels, and the rates of recovery were even more rapid than those seen in wild-type (IRBP+/+) mice.

Animals↗
PubMedDate not supplied

Cobalt ions inhibit negative feedback in the outer retina by blocking hemichannels on horizontal cells.

In goldfish, negative feedback from horizontal cells to cones shifts the activation function of the Ca2+ current of the cones to more negative potentials. This shift increases the amount of Ca2+ flowing into the cones, resulting in an increase in glutamate release. The increased glutamate release forms the basis of the feedback-mediated responses in second-order neurons, such as the surround-induced responses of bipolar cells and the spectral coding of horizontal cells. Low concentrations of Co2+ block these feedback-mediated responses in turtle retina. The mechanism by which this is accomplished is unknown. We studied the effects of Co2+ on the cone/horizontal network of goldfish retina and found that Co2+ greatly reduced the feedback-mediated responses in both cones and horizontal cells in a GABA-independent way. The reduction of the feedback-mediated responses is accompanied by a small shift of the Ca2+ current of the cones to positive potentials. We have previously shown that hemichannels on the tips of the horizontal cell dendrites are involved in the modulation of the Ca2+ current in cones. Both the absence of this Co2+-induced shift of the Ca2+ current in the absence of a hemichannel conductance and the sensitivity of Cx26 hemichannels to low concentrations of Co2+ are consistent with a role for hemichannels in negative feedback from horizontal cells to cones.

Animals↗
PubMedDate not supplied

Organization and transcription of canine (CAC)n sequences.

Twenty-nine canine DNA samples, including samples from 11 members of an autosomal recessive retinal degeneration pedigree, were fingerprinted with a (CAC)n probe using a chemiluminescent method. Sixteen polymorphic bands were identified in the two pedigrees. Nine polymorphic bands were scored in one pedigree; seven polymorphic bands were scored in the other pedigree. None were sex-linked. Hybridization of the (CAC)n probe to total and poly A+ RNAs from different canine tissues revealed a prominent polyadenylated message at 6 kb that was present in the retina, spleen, and kidney but not in the liver or heart.

Animals↗